In WO 2013/007654 A1 a prior art IGBT having trench gate electrodes is described. The IGBT comprises an (n-) lowly doped drift layer between an emitter side and a collector side. Towards the collector side, a collector layer is arranged on the drift layer. A p doped base layer is arranged between the drift layer and the emitter electrode. The base layer is completely separated from the drift layer by an n doped enhancement layer, which is higher doped than the drift layer. Due to the enhancement layer losses are lowered. An n doped source region is arranged on the base layer towards the emitter side and electrically contacts the emitter electrode. The source region has a higher doping concentration than the drift layer.
At least two first trench gate electrodes are arranged lateral to the base layer and extend deeper into the drift layer from the emitter side than the base layer. Each first trench gate electrode is separated from any surrounding layer or region (base layer, the enhancement layer and the drift layer) by a third insulating layer.
A first channel is formable from the emitter electrode, the source region, the base layer and the drift layer between two first trench gate electrodes. The source region is arranged between two first trench gate electrodes. The trench gate electrodes may have any design well-known to the experts like cellular design, full or part stripes. The first trench gate electrodes are insulated from the emitter electrode by another insulating layer.
A “grounded” gate electrode comprises a second trench gate electrode and a fifth electrically conducting layer, both of which are grounded, i.e. they are on the potential of the emitter electrode. The second trench gate electrode is arranged lateral to the base layer and extends deeper into the drift layer than the base layer. The second trench gate electrode is separated from the layers surrounding it, i.e. the base layer, the enhancement layer and the drift layer, by a third insulating layer.
An electrically conductive fifth layer covers and laterally extends outside the second trench gate electrode at least to a region above the base layer. The second trench gate electrode is mechanically and electrically connected to the electrically conductive layer. The electrically conductive layer contacts the second trench gate electrode at a recess and is thereby grounded.
The second trench gate electrodes have the potential of the emitter electrode and thereby, restrict the controllable trenches to the designed active channel regions. The use of a T-trench shorted to the emitter electrode in the region between gated (active) trenches, provides the required blocking capability by preventing in blocking state the electric field line crowding at the active trench cell.
According to WO 2006/125330 A1 a p doped bar having higher doping concentration than the p base layer may be arranged such that the source regions, the base layer, the first and second trench gate electrode terminate at the bar. The bar extends to the surface of the wafer. The bar extends in a plane parallel to the emitter side perpendicular to the direction, in which the first source regions attach the first trench gate electrodes or in which the enhancement layer separated the base layer from the second trench gate electrode. The p+ bar shall be biased, i.e. electrically connected to the emitter electrode hence providing an alternative path for the holes during dynamic avalanche. In this way the hole current during dynamic avalanche is partially flowing through the p+ bars allowing the cell to reach the critical latch up current for higher values of the total turn-off current. However for such a case, the highly doped p bar would create an undesired low impedance path between the emitter electrode in the active cell and the base layer on the side of the second gate unit enabling the drainage of the holes accumulated during the conduction state in the area of the dummy trenches (second gate unit). As a consequence a device with biased p+ bars would have higher conduction losses than a device with fully floating p+ bars.